Air mass
An air mass is a large body of air that exhibits nearly uniform temperature and humidity characteristics, often extending hundreds of miles. The characteristics of an air mass are influenced by its geographic source region; for instance, air masses originating from polar areas tend to be cold and dry, while those from tropical oceans are warm and moist. Meteorologists classify air masses based on their moisture content and temperature, distinguishing between continental (formed over land) and maritime (formed over water) air masses. Specific types include arctic, polar, and tropical air masses, each with distinct temperature and moisture profiles. As air masses move, they can interact with one another at boundaries called fronts, leading to various weather phenomena such as thunderstorms, snowstorms, and hurricanes. Cold fronts, warm fronts, stationary fronts, and occluded fronts are all classifications that describe how air masses interact, affecting local weather patterns. Understanding air masses is crucial for predicting weather changes, as their movement and interaction can result in significant atmospheric events.
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Air mass
An air mass is a large body of air with nearly uniform temperatures and humidity. An air mass can cover an area hundreds of miles wide and assumes the characteristics of the geographic surface below. For example, an air mass originating in the polar regions is cold and dry, while an air mass over the surface of the tropical oceans is warm and moist. As air masses move around the planet, they can take on the characteristics of new terrain and encounter other air masses. The interaction between air masses has a direct effect on the earth's weather and is responsible for conditions ranging from snowstorms to hurricanes.
Background
Meteorologists classify air masses by the moisture content of the air and the temperature of the surface over which they originate. The area over which an air mass originates is called the source region. In the United States, the National Weather Service classifies air masses as continental if they form over land and as maritime if they form over water. Drier continental air masses are designated by a lowercase c, while more moist maritime air masses are designated with an m.
Each of these air masses can be further classified by the surface temperature of the ground below. Arctic air masses (A) form over the extreme northern and southern regions of the planet, such as Greenland or Antarctica. Polar air masses (P) originate in the higher northern and southern latitudes, such as Canada or Siberian Russia. Polar air masses are cold, but not as cold as arctic air. Tropical air masses (T) are warm and hot and originate in the tropical latitudes nearer to the equator.
These designations can be combined to describe five specific air masses. A continental arctic (cA) air mass is bitterly cold and dry. A continental polar (cP) air mass is also dry, but not as cold. A maritime polar (mP) air mass is cold with a higher moisture content. A continental tropical (cT) air mass—such as those found over deserts—is warm and dry, while a maritime tropical (mT) air mass is warm and moist.
Arctic air can form over the ocean, however, the waters of the northern polar region remain ice-covered all year; as a result, there is no maritime arctic classification. Some classification systems include hot equatorial air masses that form along the oceans at the equator. These air masses rarely affect land.
As air masses move across the planet, they can acquire new characteristics from the terrain below and change classification. For example, a cold, dry continental arctic air mass can move out over the ocean, pick up more moisture and warmth, and become a maritime polar air mass.
Overview
Air masses move based on the jet steam, a steering flow of air in the upper atmosphere. When moving air masses meet, the air does not merge, but rather interacts at boundaries known as fronts. Colder air is heavier and displaces warmer air, causing it to rise. Air cools as it rises. This can cause water vapor in the air to condense into water droplets, forming clouds and precipitation.
A front is classified according to the temperature change based on the motion of the advancing air mass. A cold front occurs when a colder air mass replaces a warmer air mass. Cold fronts usually move from northwest to southeast and can be accompanied by an area of low pressure. The steeper vertical slope of a cold front pushes the warmer air up more quickly. This can result in a line of showers out ahead of the front. Depending on the amount of moisture in the air, this upward motion can cause a significant amount of precipitation. The advance of a cold front into warmer air is responsible for creating summertime thunderstorms.
A warm front occurs when warmer air overtakes and replaces a colder air mass. The vertical slope of a warm front is gentler than that of a cold front, so the air rises more gradually along its boundary. Warm fronts can produce a heavy layer of low clouds and a widespread area of light precipitation if the air has enough moisture.
A stationary front marks the boundary between two air masses that are not moving. Stationary fronts can bring days of cloudy or foggy weather, possibly accompanied by precipitation. When one of the air masses begins to move, the stationary front will be classified as a warm or cold front, depending on the advancing air mass. An occluded front is a less common type of weather system that occurs when a warm air mass is caught between two colder air masses. The colder and faster-moving of the two cold air masses undercuts the warm air, forcing it up and above the colder air masses. Precipitation accompanying an occluded front is similar to that of a cold front.
During the winter months in the Northern Hemisphere, the jet stream dips, allowing polar or arctic air to move south. If these cold, dry air masses meet moist tropical air from the south, a cold front can be formed, lifting the warm air up and causing precipitation. If the air contains sufficient moisture and the temperatures in the upper atmosphere and near the ground are cold enough, these conditions could result in a significant snow or ice storm. In the United States, a continental polar air mass moving over the waters of the Great Lakes can cause a phenomenon known as lake-effect snow.
When a cold air mass encounters a wave of thunderstorms over warm tropical waters, strong updrafts can occur, pulling large amounts of moisture into the atmosphere. If conditions are right, this process can set off a chain reaction leading to the formation of a tropical cyclone. In the Atlantic Ocean, these storms are called hurricanes. Tropical cyclones are potentially deadly storms that can span hundreds of miles with sustained winds of more than 74 miles per hour (119 kilometers per hour).
Bibliography
Ackerman, Steven A., and John A Knox. "Air Masses and Fronts." Meteorology, 4th ed. Jones & Barlett Learning, 2015, pp. 272–97.
"Air Masses." National Center for Atmospheric Research, 5 June 2023, scied.ucar.edu/shortcontent/air-masses. Accessed 21 Nov. 2024.
"Air Masses and Fronts." Cooperative Institute for Meteorological Satellite Studies, cimss.ssec.wisc.edu/wxwise/class/frntmass.html. Accessed 21 Nov. 2024.
Means, Tiffany. "Types of North American Air Masses." ThoughtCo., 21 Aug. 2017, www.thoughtco.com/types-of-north-american-air-masses-3443886. Accessed 21 Nov. 2024.
Nelson, Stephen A. "Tropical Cyclones (Hurricanes)." Tulane University, 2014, www.tulane.edu/~sanelson/New‗Orleans‗and‗Hurricanes/tropical‗cyclones.htm. Accessed 21 Nov. 2024.